In the papermaking process, calcium oxalate scale often forms on process equipment during the bleaching/delignification of pulp by chlorine, caustic soda, chlorine dioxide, hypochlorite and peroxide. Usual areas of scale build-up are on washer drum face wires; in washer vats; in stock lines and pumps; in filtrate tanks, lines, and pumps; on extraction screens; and in treatment towers. The formation of calcium oxalate scale provides an economic hardship on mills principally because of lost production due to decreased bleaching/delignification efficiency and equipment downtime associated with the removal of scale.
One option available to pulp mills to attempt to prevent this scale build-up has been the use of continuously fed scale control agents. Historically, these additives have included polyphosphates, organic phosphonic acid, lignosulfonates, and various water soluble polycarboxylates. However, recent industrial trends to recycle water in order to reduce bleach plant effluent volumes for environmental reasons and the use of calcium carbonate laden paper machine white water as bleach plant process water are expected to increase the frequency and severity of calcium oxalate scale formation. Moreover, increases in bleach plant hardness and oxalate levels associated with these trends have caused some of the conventional scale control agents to fail because of their inability to function under conditions which promote extensive scale formation, namely high levels of calcium and oxalate.
It is thus one object of this invention to provide a method for the control of calcium oxalate scale in paper process system. It is a further object of this invention to provide a method which adequately controls the formation of calcium oxalate scale even in aqueous systems having high calcium and oxalage levels.